While you are correctly using the declaration, it is important to note that transitions can only be applied to numeric values. This is because transitions calculate intermediate steps between two transitioning values. Unfortunately, there are no steps for a value like three-quarters-fixed-one-quarter-static.
The working draft for the transition shorthand provides guidance on what can realistically be animated.
In essence, transitioning this specific property will not be possible using CSS transitions. The most likely solution would involve JavaScript at this point. However, without seeing the actual code you are working with, it is difficult to provide a definitive answer.